I routinely use two separate Google accounts. One for personal use, and one for business. This was a huge pain until just recently when I discovered a great way to switch from one to the other. I now use a Greasemonkey script called “Google Account Multi-Login.”
This script basically replaces the “Sign Out” link on Google pages with a pull-down menu of your accounts. To change to a different account, you simply click the menu and select the account you’d like to switch to. A few seconds later your new page is loaded, and you’re in your other account. Simple as that! Passwords are stored, and there are basic options to add or remove accounts.
If it’s purely Gmail accounts you’re concerned about, there may be a second option for you. There’s a Firefox Add-on called Gmail Manager that lets you manage multiple Gmail accounts and receive new mail notifications. It also allows you to view account details – including unread messages, saved drafts, spam messages, labels with new mail, space used, and new mail snippets.
I wasn’t as impressed with the Gmail Manager, but it mostly depends on your individual needs. If you have more than one Google account, I’d highly recommend trying out the Google Account Multi-Login.